1. Field of the Invention
The present invention relates generally to an apparatus for inspecting the surface condition of an object, and more particularly to an inspection apparatus suitable for detecting marks formed on the sample surface.
2. Description of Background Art
Heretofore, it is known an inspection apparatus using the Schlieren optical system for detecting a surface condition (e.g. waviness, dimples, projections, cleaning imperfection, buff damages etc.) of mirror wafers or the like used for manufacturing semiconductor integrated circuits. The Schlieren optical system is described, for example, in a publication "NEEDS AND SEEDS FOR OPTICAL MEASUREMENT" (Measure-control association corporation/Reserch study committee for the application of light to measurement technology, 1990, The Corona Inc.).
The Schlieren optical system is one of representative systems for indicating variations of the refractive index and the reflectivity due to the irregularity of a sample surface as differences in brightness. In this optical system, light from a point source is converted into collimated light via an optical element (lens), the collimated light is irradiated to the sample surface perpendicularly thereto, then the light reflected thereby is converged via an optical element (lens), and finally a reflected image is observed by the naked eye, a camera or the like behind the knife edge.
According to this optical system, when reflected by the sample surface the light is scattered at a portion of the sample surface having irregularities thereon, and the scattered component of the reflected light impinged on the knife edge is cut off. As the result of which, a portion of the image corresponding to the scattered component cut off by the knife edge becomes dark behind the knife edge, and the other portion becomes bright. Since this light/dark (i.e. brightness) pattern corresponds to the surface condition of the sample, it is possible to observe the surface condition of the sample.
By the way, in the Schlieren optical system wherein any knife edge is not used and thereby no scattered component is cut off, since the whole image becomes bright, the observation is carried out by obtaining a bright/dark pattern having small quantity of light. There is therefore a problem that the light/dark pattern is very hard to observe for lack of its contrast.
On the other hand, it is known another type of such a Schlieren optical system which has not any knife edge and in which the observation of a surface condition of a sample is carried out at a selected region having a relatively high contrast by setting its focus slightly off the point.
However, these inspection apparatus of the prior art have following problems.
That is, in the inspection apparatus using the former type Schlieren optical system, although it is possible to obtain a light/dark pattern corresponding to the surface condition of the sample behind the knife edge, the contrast of the light/dark pattern is low since it contains much scattered component which passes through the opposite side of the knife edge and therefore cannot be cut off by it. In addition, since the range which can be observed behind the knife edge is part of the sample surface and not the whole thereof, the sample should be turned one revolution around its optical axis in order to observe the whole sample surface.
On the other hand, in the inspection apparatus using the latter type Schlieren optical system in which the observation plane is moved along the optical axis, the contrast obtained is very low. In addition, although it is possible to vaguely discriminate an area including irregularities, it is impossible to clearly discriminate the magnitude of the depth and height of the area.